1. Field Of The Invention
The present invention relates to a powertrain component, such as a valve actuation mechanism in an internal combustion engine. More particularly, the invention relates to a component having a hard, wear resistant coating of an amorphous hydrogenated carbon film formed thereupon.
2. Related Art Statement
In most internal combustion engines, there are various powertrain components. Illustrative is a valve actuation mechanism, which includes a valve lifter or tappet positioned between a cam and a valve stem or a rocker arm. As a lobe of the cam moves in relation to the valve lifter, rotational movement of the cam lobe is translated into linear movement of the valve lifter and the valve stem which moves in reciprocal cooperation therewith. Whether or not the powertrain component operates in an oil-starved environment, traditional problems of noise, vibration, and wear have resulted from frictional and normal forces generated between adjacent interacting surfaces, particularly over prolonged periods at high operating temperatures.
Depending on the design of the internal combustion engine, the valve lifter may reciprocate within a guiding aperture formed in a cylinder head. In such situations, the problem of wear caused by adhesion between adjacent surfaces may arise.
Illustrative of approaches to such problems is U.S. Pat. No. 4,909,198, which issued on Mar. 20, 1990. That reference discloses an aluminum alloy valve lifter with an iron-carbon coating sprayed thereupon. Such coatings, however, differ chemically, structurally, and in the method of formation from the invention disclosed and claimed below. The disclosure of U.S. Pat. No. 4,909,198 is herein incorporated by reference.
The notion of selectively applying a synthetic diamond or diamond-like material on certain engine components is disclosed in U.S. Pat. No. 4,974,498 which issued on Dec. 4, 1990. The '498 patent refers to the application of films which are primarily crystalline in nature. Such films are disclosed as having protective utility when formed on specific engine components, such as pistons, piston rings, connecting rods, and crankshaft bearings. Those coatings also are different in composition and morphology from the invention disclosed and claimed below. Moreover, synthetic diamond films tend to be abrasive and may not be generally applicable to powertrain components and engines where there are rubbing contacts.
Formation of carbonaceous films on substrates can be accomplished by several known processes. Such processes include radio frequency (RF), ion beam and microwave plasma chemical vapor deposition (CVD) techniques. If applied satisfactorily, such coatings could reduce friction and wear. Depending on the technique used, several problems may remain. They include delamination of the film in an operating environment, which may be occasioned in part by compressive stresses engendered during deposition at the film/substrate interface. In general, the thicker the film, the higher the compressive stresses engendered during film formation. If such stresses are excessive, delamination may result. Other problems may arise from chemical incompatibility of the substrate and the coating.
As an example, aluminum and its alloys have been among those substrates with which conventional deposition techniques have yielded only marginal results. This is because, in part, aluminum carbides tend to be water soluble and unstable, especially in conditions of prolonged exposure to high humidity. Accordingly, the direct application of carbonaceous films to an aluminum-containing substrate may be intrinsically problematic.
In order to perform their protective role, films have to adhere persistently to the substrate. To do this, the adhesive forces need to overcome the high internal stresses engendered in the film which may otherwise cause the films to delaminate from the substrate. As with other properties, the adhesion of protective films is dependent on the preparation method and obviously the substrate on which they are deposited.
Against this background, the need has arisen to devise a powertrain component and method for preparing a substrate-coating structure which has a reliably adherent hard, wear resistant film, while accommodating compressive stresses generated during film formation and avoiding problems associated with chemical incompatibility between the film and the substrate.